In digital signal processing systems in which an analog signal is converted to pulse code modulated PCM (e.g., binary) form, processed in PCM format and then converted back to analog form, it is desirable to constrain the number of bits per sample to the smallest number consistent with the desired quantization resolution and dynamic range of the signal. In fact, however, the number of bits per sample may increase as the samples progress through the digital system and the required bit capacity of the digital-to-analog converter may exceed the sample bit width of the analog to digital converter. This is particularly true in processing video signals. Consider, for example, a digitaly processed luminance signal which represents a black and white video image. The average brightness of the image is dependent upon the D.C. or average amplitude of the luminance signal. If the dynamic range of the luminance signal is exactly equivalent to the maximum value which can be represented by the bit width of the processing system, brightness control cannot be digitally implemented without clipping the signal peaks, on the one hand, or expanding the system bit width on the other hand. In general both alternatives are undesirable since they either compromise performance or increase system cost.
Consider also a pix-in-pix video system wherein video images from separate sources are simultaneously displayed on exclusive portions of a single display screen. Generally, it is desirable that the average brightness of both images be approximately equal so that the viewer's attention is not significantly attracted to only one of the two images. In order to insure that the average brightness of the two images is approximately equal, typical pix-in-pix receivers will cause the two signals representing the two images to be clamped to like D.C. reference values.
It is an object of this invention to provide a digital D.C. level control in a digital and analog video processing system without expanding the system bit width or compromising the signal quality.
It is a further object of the invention to provide a clamp circuit for clamping the average D.C. value of two video signals to the same value.